Constructive modelling of articles

ABSTRACT

A method for constructive modelling of articles including prostheses and anatomical pathology wherein CT scan data is computed to construct a plurality of two dimensional cross sectional images along one axis and the two dimensional image data is computed to create three dimensional coordinate data sets for the articles to be modelled. The three dimensional data sets are then computed to obtained spaced parallel two dimensional image data sets in a second plane of the article and the reconstructed two dimensional image data sets are employed in a constructive modelling process to produce a three dimensional model of the article or part thereof.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 08/596,237 filed Apr. 25, 1996, now U.S. Pat. No. 5,741,215,entitled "Stereolithographic anatomical modelling process" which in turnis a §371 of PCT/AU94/00536, filed Sep. 12, 1994.

FIELD OF INVENTION

This invention is concerned with methods and apparatus for formingconstructive modelling of articles including, but not exclusively,models of anatomical pathology and prosthetic devices.

BACKGROUND ART

A variety of methods and apparatus for three dimensional modelling ofarticles including prosthetic implants are known. Many of thesetechniques employ digitised information from CAD-CAM design systems ordata captured and/or reconstructed from a variety of reflection and/ortransmission scanning devices.

Such scanning devices include laser and acoustic reflection apparatusand various types of transmission apparatus including X-ray, magneticresonance imaging (MRI) including spiral scan MRI, magnetic resonanceangiography (MRA), positron emission (PET or SPECT) as well asultrasonic radiation. Typically, data is captured by scanning a seriesof spaced parallel planes which may then be combined by computertomography (CT) techniques to reconstruct a two or three dimensionalprojection of the article so scanned.

Modelling of anatomical pathology using computed tomography data is wellknown for pre-operative planning and rehearsal of procedures and in themanufacture of prosthetic devices.

U.S. Pat. No. 4,436,684 describes a non invasive method for formingprostheses of skeletal structures for use in reconstructive surgery.Three dimensional coordinate data is obtained directly from the digitaldata generated by the computed tomographic information. The threedimensional coordinate data is then utilised to generate threedimensional cylindrical coordinates which are specified relative to anorigin which is coincident with the origin of a coordinate system usedin a sculpting tool apparatus to specify the spatial location of acutting tool relative to a workpiece rotating on a turntable.

Due to difficulties in supporting the workpiece however it is generallynot possible to sculpt an entire three dimensional model of an article,rather, this system is employed to construct models of portions ofskeletal structures to act as male or female mould surfaces forconstruction of prosthetic inlays or onlays.

This apparatus and system however cannot construct a hollow model havingfaithfully reproduced external and internal surfaces and structuralfeatures.

U.S. Pat. No. 4,976,737 describes a method of forming a prostheticdevice by employing the apparatus and method described in U.S. Pat. No.4,436,684 to form a template which may be used directly or indirectly tocreate a mould surface for moulding a polyurethane impregnated Dacron(Trade Mark) prosthesis. This document describes in detail a "mirrorimaging" technique to generate digital data for reconstruction of amissing, damaged or deformed portion of a skeletal structure bytransferring image data prom one side of an axis of symmetry to another.

Stereolithographic modelling of engineering components from UV sensitivecross-linkable acrylic polymers using CAD/CAM digital data is known. Ofmore recent times, the use of stereolithography for creation of threedimensional models of bony structures has been reported.

Stereolithographic modelling of anatomical pathology to provide a farmore accurate means for physicians and surgeons to examine the conditionof a patient for the purposes of diagnosis and for surgical procedures.Rather than rely upon say a solid model representing external featuresalone (as with U.S. U.S. Pat. No. 4,436,684, with or without twodimensional tomographic images, stereolithographically reproducedrepresentations of anatomical pathology provide an almost exact replicaof both internal and external features of a region under consideration.

Moreover, such stereolithographically reproduced models permit surgicalprocedures to be pre-planned and rehearsed with a great deal ofprecision to minimise risks and trauma and should enable a means forpreparing accurate prostheses for surgical repair of defects or inreconstructive surgery.

One of the difficulties in reconstructing three dimensional co-ordinatedata from X-ray tomographic scans is that in order to minimise theamount of radiation to which a patient is exposed, the tomographic"slices" are relatively widely spaced and complex computer programmesare required to reconstruct this scanning data. Typically a "slice" isabout 1.5 mm in thickness and "slice" data is obtained at about 1.0 mmintervals. A scan of an adult human skull may thus comprise 70-80tomographic "slices".

A comparison of three dimensional CT image reconstructions using adestructive mechanical milling process and a constructivestereolithographic modelling process is described in "Paediatriccraniofacial surgery: Comparison of milling and stereolithography for 3Dmodel manufacturing", Pediatr. Radiol. (1992) 22: 458-460. This articleaddresses the limitations of the milling process and concludes thatwhile stereolithography is extremely expensive by comparison, "The sliceoriented construction of the model corresponds well with thecross-sectional imaging methods and promisses (sic) for the future adirect transfer from image slice to object slice."

Similar mechanical and stereolithographic modelling processes aredescribed respectively in "Computed-Aided Simulation, Analysis, andDesign in Orthopaedic Surgery", Orthopaedic Clinics of NorthAmerica--Vol 17, No. 4, October 1986 and "Solid models for CT/MR imagedisplay: accuracy and utility in surgical planning", /SPIE Vol 1444Image Capture, Formatting and Display (1991): 2-8.

Both of the references referred to immediately above describe in detaila computed tomography slice processing technique utilising proprietarysoftware to trace all bone boundaries in the image volume afterempirically determining the threshold for cortical bone. The algorithm,after exhaustively searching each image, locates the inner and outeredges of cortical bone objects and generates a contour volume data set.This data set is passed to a reformat program to generate the SLA buildfile containing information necessary to operate the stereolithographyapparatus.

In both of these references, the technique requires that the exhaustivecontour descriptions must be replicated four times to provide a finishedlayer of 0.25 mm in thickness. This repetition is necessary toreconstruct the CT axial resolution as one CT slice equals four SLAlayers.

In transforming contour data to CAD data, a number of algorithms areavailable. A simple algorithm uses simple thresholded segmentation toproduce voxel faces as paired triangles. A more complex technique usesthe "Marching Cubes" algorithm which interpolates slices to yield asurface composed of sub-voxel polygons. The "Marching Cubes" algorithmis described in "Two algorithms for the three-dimensional reconstructionof tomograms", Med Phys. 15(3): 320-7, and "Marching Cubes: a highresolution 3D surface construction algorithm" Computer Graphics.21:163-169.

A suitable proprietary algorithm which has been used with the method ofthe present invention is "ANALYZE" three dimensional imaging software(v.2.4) by The Mayo Biomedical Imaging Resources.

U.S. Pat. No. 5,357,429 describes initial imaging of an object at avariety of non perpendicular angles with respect to a longitudinal axisof the object and, after construction and inspection of respective threedimensional images representing each set of parallel two dimensionalimages, a decision is made to select one set of parallel two dimensionaldata sets to generated directly the build of a three dimensional model.Accordingly, the upright axis along which the model is built isorthogonal to both the build and scan planes which are parallel to eachother.

U.S.Pat. No. 5,452,407 concerns a method for converting image data tovector data wherein the two dimensional image data sets are computed inparallel planes at closer intervals than planes in which the twodimensional cross-sectional images are obtained by scanning.

Neither of U.S. Pat. No. 5,357,429 or U.S. Pat. No. 5,452,407contemplate utilising computed two dimensional image data sets in planesother than parallel to the original scan planes.

Other prior art references dealing with image reconstruction and/ormodelling utilising scan image data are U.S. Pat. Ser. Nos. 5,299,288,5,554,190, 5,454,383, 5,443,510, 5,373,860, 5,358,935, 4,936,862,4,902,290, 5,127,037, 5,612,885, 5,487,012, 4,589,992, 4,953,087,4,976,737, 5,217,653 and 5,231,470. International publicationsWO,A,9208200, WO,A,8910801 and WO,A, 9106378 as well as European patentapplication EP,A,574099 describe similar processes.

None of the above referenced processes describe modelling of articles ina plane which is not parallel to the original planes in which scanningoccurs.

In order to control the apparatus for constructive modelling, contourinformation determined from tomograms this may be introduced into a CADsystem to generate surface models composed of triangular approximationswhich is the standard interface between a CAD system and the modellingapparatus.

In addition to contour construction, the region between the inner andouter boundaries must be defined by hatch vectors to enable the solidregion to be formed by cross linking of monomer in a predefined regionin the monomer bath. By generating not only the contours, but alsohatchings with different densities it is possible to produce differentstructures to represent differing structural densities of a scannedarticle.

Of more recent times however, there has been reported a more directtechnique in "Medical Applications of Rapid Prototyping Techniques":201-216.

This system addresses both the support generation and interpolationproblems of earlier systems and is able to create directly from the CTscans the SLA files of both the model and its support structures in amuch shorter time.

While it may be advantageous to utilise direct layer interfacing suchthat the most accurate directions of the input data in the scanningplane are produced on the most accurate directions of thestereolithography apparatus, the lack of true three dimensional datarequires, as with the prior art systems, that the orientation of thepart in the stereolithography apparatus should be the same as theorientation during the patient scanning operation.

There are a number of serious disadvantages associated with constructivemodelling of articles in the same orientation as conventional patientscanning orientation.

As models are built up from successive 0.25 mm layers of polymerisedresin in, say a stereolithographic modelling process, an upright modelwill take substantially longer to manufacture than a horizontallyorientated model. For example, a 50 mm diameter cranial defect wouldrequire about 200 layers of polymerised material when in an uprightposition as against about 10-20 layers when the model is built in ahorizontal orientation. Costs of model production could therefore besubstantially reduced if manufacturing time could be reduced byselective orientation of models for constructive modelling.

Moreover, selective orientation of models during manufacture wouldpermit a plurality of objects to be simultaneously modelled andorientated in the most efficient manner.

A further disadvantage is that with model construction limited to asingle orientation, it is not possible to selectively orientate themodel for construction to minimise the extent of support structure whichmust subsequently be removed from the completed model in saystereolithography, laser sintering, 3D printing or like processes.

In all prosthetic implant surgery it is essential that a very close fitis obtained between the prosthesis and the tissue to which it isattached if an effective bond is to be obtained from tissue growth.Accordingly, there is a need for a much more accurate method forconstruction of prosthetic implants, both for hard and soft tissueregions, to ensure an initial accurate fit and accurate contour to avoidintraoperative delays while adjustments, contour changes or prolongedattachment procedures are undertaken.

It would also be advantageous in arterial and vascular surgery toprovide complex branched prostheses which require attachment to bloodvessels at the free ends of the prosthesis rather than having toconstruct the prostheses from tubular sections of varying diametersintraoperatively as is the case at present.

Similarly in rapid prototyping of industrial or other models,particularly those comprising a plurality of separate partes, it isessential to achieve accurate and cost-effective modelling to reducedevelopment time.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide an improved method forconstructive modelling of articles utilising digitized informationreconstructed from conventional scanning devices.

It is another aim of the present invention to provide improvedimplantable prostheses formed according to the method of the invention.

It is yet a further aim of the invention to provide an improved methodfor implantation of prostheses formed according to the method of theinvention.

Although the invention described herein will be exemplified by referenceto stereolithographic modelling of anatomical pathology, it readily willbe obvious to a person skilled in the art of constructive modelling thatthe process is equally applicable to modelling of any articleswhatsoever provided they are dimensions permitting scanning byconventional scanning equipment.

Similarly, the process is equally applicable to any constructivemodelling process wherein the model is built up in successive layerscorresponding to reconstructed two dimensional image scan data. Suchprocesses may include but are not limited to stereolithography, lasersintering of powdered resins, lamination of films or sheets, threedimensional printing of curable or polymerisable resins or the like.

Constructive modelling is to be distinguished from destructive modellingprocesses wherein a mass of material is eroded by mechanical, chemicalor electrical means to form an article having at least an externalcontour of predetermined shape.

According to one aspect of the invention there is provided a method forconstructive modelling of articles, said method comprising the stepsof:--

inputting into a data storage means scanning data relating to internaland/or external surfaces of an article;

computing the stored scanning data according to a predeterminedalgorithm to reconstruct a plurality of two dimensional cross-sectionalimages of the article;

computing said plurality of two dimensional cross-sectional imagesaccording to a predetermined algorithm to generate a three dimensionalcoordinate data set for the article;

and generating a three dimensional representation of said article by aconstructive modelling process using selected sequential two dimensionalcoordinate data sets computed in preselected planes from said threedimensional coordinate data set.

Suitably said scanning data comprises digitised X-ray, MRI, MRA, PET,SPECT acoustic or other computed tomographic data.

If required the stored scanning data may be computed to reconstruct aplurality of two dimensional cross sectional images of an existingarticle.

Alternatively the stored scanning data may be computed to reconstruct aplurality of two dimensional cross-sectional images of a selected regionof the article scanned.

If required, the two dimensional images of said selected region may begenerated by direct computation of image data obtained from acorresponding region on an opposite side of a symmetrical axis of areconstructed two dimensional image.

Alternatively said two dimensional images of said selected region may begenerated by overlaying, in the selected region, an image obtained froman opposite side of a symmetrical axis of a reconstructed twodimensional image, assigning respective values to the image dataobtained from opposite sides of said symmetrical axis and adding orsubtracting the assigned values to obtain two dimension image data forthe selected region only.

If required, the two dimensional data obtained for the selected regionmay be manipulated to obtain a best fit or enhanced fit with theselected region.

Preferably said method includes the step of simultaneously modelling afirst portion of an article with a contour adapted to receive a secondportion of said article; and, a second portion of said articlecomplementary to said contour.

The model may be constructed directly by a suitable constructivemodelling technique.

Alternatively the model may be constructed indirectly by forming a mouldfrom the model so formed.

Preferably however the model may be constructed indirectly bymanipulation of the scanned data, the reconstructed two dimensionalimages or the three dimensional coordinate data set to form a mould ormould surface from which the model may be moulded or otherwise formed.

The model may comprise any suitable material compatible with aconstructive modelling process.

In the event that the model is intended as a prosthesis, it may beconstructed directly or indirectly from a biocompatible or bio-inertpolymeric organic compound such as acrylic polymers and co-polymers,polyesters, polyolefins, polyurethanes, silicon polymers andco-polymers, vinyl polymers and co-polymers, halogenated hydrocarbonssuch as Teflon (Trade Mark), nylons etc. or even proteinaceousmaterials.

Preferably the prosthetic implant is constructed directly byconstructive modelling of a polymerisable or cross-linkableproteinaceous material.

If required the prosthetic implant may be constructed indirectly from aninorganic compound such as hydroxyapatite, ceramics or like materials.

Suitably the prosthetic implant is porous or comprises porous regions topermit bonding by tissue migration.

If required the prosthetic implant may be impregnated with tissue growthstimulation factors such as bone morphogenetic protein or the like.

The prosthetic implant may include mounting or attachment means tofacilitate attachment to adjacent anatomical pathology.

According to yet another aspect of the invention, there is provided amethod for the surgical implantation of a prosthesis comprising thesteps of:--

topographically scanning a region of anatomical pathology;

inputting scanning data so obtained into a data storage means;

computing the stored scanning data according to a predeterminedalgorithm to reconstruct a plurality of two dimensional cross-sectionalimages of the anatomical pathology;

computing said plurality of two dimensional cross-sectional imagesaccording to a predetermined algorithm to generate a three dimensionalcoordinate data set for the anatomical pathology;

and generating a three dimensional representation of said anatomicalpathology or a mould surface therefor by stereolithographic modelling ofa cross linkable liquid polymer using selected sequential twodimensional coordinate data sets computed in preselected planes fromsaid three dimensional coordinate data set; and surgically implanting ina patient a prosthesis obtained directly or indirectly therefrom, saidprosthesis being characterised in having a close fit with connectivetissue and contours appropriate for the implant site.

Preferably said three dimensional representation includes a region ofanatomical pathology surrounding or adjacent to the region of anatomicalpathology being modelled, said surrounding region providing a templatefor accurate fit of a prosthesis so obtained.

BRIEF DESCRIPTION OF THE INVENTION

In order that the various aspects of the invention may be more fullyunderstood and put into practice, reference will now be made to variouspreferred embodiments illustrated in the drawings in which:--

FIG. 1 shows schematically a data flow chart from capture through tooperation of the SLA apparatus.

FIG. 2 shows a cranial defect and a prosthesis therefor.

FIG. 3 shows a cross sectional view of a cranio-plastic implantationaccording to one method.

FIG. 4 shows a variation on the method of FIG. 2.

FIG. 5 shows a variation in the means of attachment of thecranio-plastic implant of FIGS. 2-4.

FIG. 6 shows a prosthetic replacement for an aortic junction aneurysm.

FIG. 7 shows an alternative method of production of a cranioplasticmodel.

FIG. 8 shows alternative methods of orientation of a model in an SLAmonomer bath.

While the preferred embodiments have been exemplified by reference tostereolithographic modelling of anatomical pathology, it is to beunderstood that the invention is applicable to modelling of any articlesby any constructive modelling process.

DETAILED DESCRIPTION

In FIG. 1, CT scan data is obtained conventionally from an X Ray, MRI,MRA, PET scanner and is processed by conventional software to produce,initially, two dimensional boundary images of say, a bony structure foreach tomographic slice.

The segmented data is then further processed by conventional contour orvoxel methods to produce a three dimensional data set for the anatomicalpathology scanned. The three dimensional data set may be manipulated byconventional CAD software if so required.

The three dimensional data set is then further processed to produceparallel two dimensional slice image data sets, which can also bemanipulated by the CAD software, before creation of the SLA build filesrequired to operate the SLA apparatus.

Once the three dimensional data set is established, two dimensionalslice data may be obtained at a selected spacing, suitably 0.25 mm tocorrespond to the model build layer thickness of the SLA apparatus.

For reasons which will be described in greater detail later, an operatoris able to chose the planar orientation of the two dimensional slicedata to optimise the SLA modelling operation rather than be constrainedto generation of SLA build file data representing two dimensional datasets only in planes parallel to the tomography scan data as with priorart systems.

Hatch file vectors may be computed from the initial two dimensionalsegmented image data but preferably hatch file vectors are computed fromthe reconstructed two dimensional image data sets for the planarorientation chosen. This avoids the need for interpolation by repetitionto create SLA build slices as with prior art systems. Moreover, thehatch file vectors are a more accurate representation of the solidstructure for each individual SLA build slice.

FIG. 2 shows a human skull 1 with a cranial defect 2.

An ideal cranio-plastic implant 3 comprises a body of bio-compatiblematerial which is shaped such that its thickness and contours aresubstantially identical to the bone which previously occupied thecraniotomy defect. The peripheral shape is substantially identical tothe peripheral shape of the defect aperture to permit a very close fit.

Accordingly, in a cranioplasty procedure, the previously manufacturedcranio-plastic implant is able to be fitted directly to the defect andsecured therein with acrylic cement, wires or screws. During such aprocedure the operating time is minimised and the inherent risk ofinfection substantially reduced as little, if any, adjustments arenecessary to adapt the implant to fit the defect.

Moreover the very close fit of the implant into the defect apertureminimises the degree of bone tissue growth required to bond with theimplant to regain maximum structural integrity of the cranial structure.

The "ideal" cranio-plastic implant described with reference to FIG. 2 isobtained by a stereolithographic method in accordance with one aspect ofthe invention.

Initially, part or all of the cranial structure of a patient is scannedto obtain X-ray or MRI computed tomography data of spaced cross sectionsin a coronal or axial plane transverse to the long axis of the body dueto the physical constraints of the scanning apparatus.

Using conventional computer software, the scanning data is segmentedaccording to tissue type to define tissue boundaries and the segmenteddata is then reconstructed as two dimensional images in the same coronalplane.

Again, using conventional computer software the data relating to thereconstructed two dimensional images is computed and interpolated byvoxel or contour means to generate three dimensional coordinate datasets which may be employed to display or print out two dimensionalimages of the three dimensional representation.

The three dimensional coordinate data sets may then be computed togenerate two dimensional image data sets at much closer intervals thanthose representing the original coronal scan planes. Moreover, these twodimensional image data sets may be generated in any desired plane eg.sagittal, medial, coronal or other planes oblique to the main orthogonalplanes.

Accordingly depending upon the position of the defect, two dimensionalcoordinate data sets may be established in one or more planes and thedata sets may be combined to provide highly accurate three dimensionalcontour and shape definition in the region of the defect, particularlyits boundaries.

FIG. 3 shows one method for generating two dimensional coordinate datasets to construct three dimensional coordinate data sets for acranio-plastic implant to be accommodated in the defect region 2.

From the closely spaced two dimensional images reconstructed from thecomputed three dimensional coordinate data sets, a two dimensional imagemay be drawn with a light pen or the like to fit the defect aperture. Inso doing the drawn image follows a visual "best fit" mode in terms ofthickness and contour. This procedure is repeated over a series ofspaced parallel planes from the top of the defect to the bottom or viceversa.

The process may be repeated in a medial plane orthogonal to the firstcoronal plane to eliminate inaccuracies in say the upper and lowerregions of the reconstructed image of the defect.

The combined image data representing three dimensional coordinate datafor a cranio-plastic implant is assigned a numerical value as is thedata representing the surrounding bone tissue. By appropriate allocationof respective values and then adding or subtracting those values, athree dimensional coordinate data set is obtained only for a structurerepresenting a cranio-plastic implant.

The data so obtained is then employed with a stereolithographicapparatus to construct a model of an implantable prosthesis from across-linkable acrylic polymer.

FIG. 4 shows an alternative method for construction of three dimensionalcoordinate data sets of a cranio-plastic implant.

In this method, two dimensional images are reconstructed from the threedimensional coordinate data sets at required planar spacings as with themethod described above.

An axis of symmetry 4 is established relative to the two dimensionalimage and the bone tissue regions on each side of the axis are assignedarbitrary values of say +1 for the left side and -1 for the right side.

A mirror image of the left side is then superimposed on the right sideimage and the numerical values of the bone tissue regions are summed.The values for the intact portions of the cranial structure arenullified leaving an image 5 having a value of +1 and representing a twodimensional cross sectional image of a plane in the region of thedefect.

As most human cranial structures are not perfectly symmetrical there maybe some misalignment of the mirror image object with the defectaperture. Using suitable graphics manipulation software or perhapssimply a light pen, corrections may be made as appropriate to align thesuperimposed image.

FIG. 5 shows yet another embodiment of the invention.

Using a graphics manipulation program, light pen or the like, thethickness of the three dimensional coordinate data for the implant maybe increased, at least toward the peripheral edges. In this manner it ispossible to build a smoothly tapered flange 6 around the periphery ofthe implant 3 to provide a more secure means of attachment to thesurrounding bone tissue and otherwise permit a greater area for tissuebonding.

Suitably, the implant shown in FIG. 5 is constructed of a somewhatporous hydroxyapatite material and is impregnated in the region offlange 6 with bone morphogenetic protein to stimulate penetration ofbone tissue into the implant.

FIG. 6 illustrates a prosthetic implant 17 to replace an aorticjunction, damaged for example by atherosclerosis and/or an aneurysm.

Using scanning data obtained from say MRI computed tomography, a complexhollow branched structure may be created directly using a flexiblecross-linkable polymeric material in a stereolithographic apparatus.Where a region of wall thickness in the patient's aortic junction isreduced or damaged by the aneurism, this can be corrected or compensatedfor by manipulating the initial or reconstructed two dimensional scanimages in a manner similar to that described with reference to FIGS. 3and 4.

Alternatively the implant may be created indirectly by creating a femalemould by a stereolithographic process, the mould having an internalsurface corresponding to the external dimensions and contours of acomputed three dimensional representation. The implant may be formed inthe mould by, say, rotational casting of a thermoplastic material or across-linkable liquid polymer.

Arterial and vascular implants constructed in accordance with theinvention have the advantage that operation time is substantiallyreduced, the number of sutured joins, sutures and suturing time is alsosubstantially reduced and the free ends of the implant are substantiallyidentical in diameter to the artery or vein to which they are to beattached.

FIG. 7 illustrates a most preferred method of creating a defectprosthesis such as a cranioplastic implant.

After establishing a three dimensional coordinate data set for a region7 surrounding the defect 2, two dimensional image data is thenreconstructed in spaced parallel planes generally perpendicular to thenotional "surface" of region 7. By orienting the reconstructed twodimensional images in this manner, highly accurate boundary definitionsare obtainable for the edge of the effect aperture as well as the crosssectional contours of region 7.

A prosthetic model for the defect is then created by a computer programand when complete, the hatch file vectors are computed for each twodimensional image spaced at intervals corresponding to each SLA buildslice.

An even more accurate two dimensional image data set may be obtained byselecting two reconstructed two dimensional image sets in planesorthogonal to each other and the "notional" surface of the defect. Theseorthogonal data sets may be combined to provide corrected twodimensional image sets in a selected plane.

With the reconstructed two dimensional image sets and the hatch filevectors obtained therefor, the SLA apparatus simultaneously buildsmodels of the defect surround region 7 and the defect adjacent eachother on the monomer bath table. When completed, the defect model 8should be a very neat fit into the aperture 9 in surround region 7. Ifrequired, the defect model 8 may be trimmed with a file or built up witha hardenable putty to improve the fit in aperture 9.

In generating the reconstructed two dimensional image data, theperipheral edge of defect model 8 may be increased to provide a body ofmaterial for subsequent trimming.

Once a satisfactory fit of model 8 in aperture 9 has been achieved, acranioplastic implant may then be manufactured from model 8 in acrylicor hydroxyapatite or other suitable material.

During a subsequent cranioplasty procedure, open wound time is minimisedby avoiding the need for intraoperative manipulation or adjustment ofthe implant. Furthermore, the highly accurate fit of the implant permitsa far more secure location of the implant with only minimal marginalgaps for bone regrowth.

FIG. 8 illustrates a particular advantage of the invention.

As prior art SLA processes are limited to modelling in slices parallelto the initial tomographic scan planes, this can lead to inefficiencies.

For example, as most bony structures such as arms, legs, hands etc. arescanned in a plane perpendicular to the longitudinal axis of the bones,SLA models of these bones are built in the direction of the longitudinalaxis.

In the SLA modelling process, layers of polymerised monomer are builtwith a thickness of about 0.25 mm. Between each layer polymerising step,the support platform in the monomer bath is lowered and positioned at apredetermined distance below the level of monomer and certain delays areencountered in these steps whilst awaiting stabilisation of the monomersurface. In the modelling of a human skull built on coronal planes forexample, the modelling process may take up to 36 hours.

According to the present invention, reconstructed two dimensional datasets for operation of the SLA apparatus may be selected in any suitableplane. Thus, in the monomer bath 10, the cranioplastic implant model 8shown in FIG. 7 may be constructed in an upright manner represented bymodel 8a or in a generally horizontal position represented by model 8b.It can be seen therefore that the time normally required to producemodel 8a can now be reduced by an amount proportional to the ratiobetween minimum and maximum supported dimensions.

Another advantageous feature of the present invention which arises fromthe choice of orientation of SLA slices of three dimensional data setsis that a plurality of models may be created simultaneously on thesurface of the support platform 11 of the SLA apparatus. By carefulselection of the plane of orientation of the reconstructed twodimensional data sets, the area of coverage of platform 11 and the modelheight may be chosen to optimise model construction time and/or tominimise the formation of support structures for "floating" artefactsduring model build steps.

Moreover, selective orientation of model build directions permitssupport structures to be confined, say, to a posterior surface of ananatomical pathology when an accurately represented model of an anteriorsurface is required or vice versa.

With the development of cross-linkable or polymerisable biocompatiblematerials, it is envisaged many prosthetic implants may be constructeddirectly by the stereolithographic modelling process rather than in atwo or three step method requiring the use of male or female moulds.

Moreover, complex reconstructive surgical procedures may be greatlysimplified by the ability to pre-form complete prosthetic implantswithout the need to obtain donor tissue from other parts of the patientsbody. Under these circumstances a prolonged series of reconstructiveprocedures may perhaps be replaced with a single procedure.

It will be readily apparent to a skilled addressee that mnanymodifications and variations may be made to the various aspects of theinvention without departing from the spirit and scope thereof and thatthe methods according to the invention may be adapted for use inconstructing a wide variety of models of anatomical pathologies as wellas other structure.

For example, although an elongate article such as, say, a turbine blademay be required to be initially scanned at spaced intervals along thelongitudinal axis, the transverse two dimensional scan data so obtainedmay be reconstructed initially as a three dimensional coordinate dataset from which new longitudinal two dimensional data sets are computed.These new longitudinal two dimensional data sets are computed inparallel planes with a spacing of say 0.25 mm corresponding with thebuild layer thickness of the SLA apparatus.

Clearly, by building an SLA model such as a turbine blade along itslongitudinal axis in a horizontal position, substantial cost savings canbe obtained compared to prior art builds on upright longitudinal axes.

What is claimed is:
 1. A method for constructive modelling of articles,said method comprising the steps of:inputting into a data storage meansscanning data relating to internal and/or external surfaces of anarticle; computing the stored scanning data according to a predeterminedalgorithm to reconstruct a plurality of parallel two dimensionalcross-sectional images of the article; computing said plurality of twodimensional cross-sectional images according to a predeterminedalgorithm to generate a three dimensional co-ordinate data set for thearticle; and generating a three dimensional representation of saidarticle by a constructive modelling process using selected sequentialtwo dimensional image data sets computed from said three dimensionalco-ordinate data set, said two dimensional image data sets beingcomputed in parallel planes which are not parallel to the planes inwhich said two dimensional cross-sectional images are obtained byscanning.
 2. A method as claimed in claim 1 wherein said two dimensionalimage data sets are computed in parallel planes at closer intervals thanplanes in which said two dimensional cross-sectional images are obtainedby scanning.
 3. A method as claimed in claim 1 wherein said scanningdata comprises digitised X-Ray, MRI, MRA, PET, SPECT, acoustic or othercomputed tomographic data.
 4. A method as claimed in claim 1 wherein thestored scanning data is computed to reconstruct a plurality oftwo-dimensional cross-sectional images of a selected region of thearticle scanned.
 5. A method as claimed in claim 4 wherein the twodimensional images of said selected region are generated by directcomputation of image data obtained from a corresponding region on anopposite side of a central axis of a reconstructed two dimensionalimage.
 6. A method as claimed in claim 4 wherein said two dimensionalimages of said selected region are generated by overlaying, in theselected region, an image obtained from an opposite side of a centralaxis of a reconstructed two dimensional image, assigning respectivevalues to the image data obtained from opposite sides of said centralaxis and adding or subtracting the assigned values to obtain twodimensional image data for the selected region only.
 7. A method asclaimed in claim 4 including the step of simultaneously modelling afirst portion of an article with a boundary defining the selected regionand a second portion of said article complementary to said selectedregion.
 8. A method as claimed in claim 1 wherein a model is constructeddirectly by a constructive modelling process.
 9. A method according toclaim 1 wherein the model is constructed indirectly by forming a mouldfrom a constructively modelled article.
 10. A method according to claim1 wherein the model is constructed indirectly by manipulation of thescanned data, the reconstructed two dimensional images or the threedimensional co-ordinate data set to form a mould or mould surface fromwhich the model is moulded or otherwise formed.
 11. A method as claimedin claim 1 wherein the constructive modelling process isstereolithography.
 12. A method as claimed in claim 1 wherein theconstructive modelling process utilises laser sintering of particulatepolymers.
 13. A method as claimed in claim 1 wherein the constructivemodelling process comprises lamination of sheet-like material.
 14. Amethod as claimed in claim 1 wherein the constructive modelling processcomprises a three dimensional printing process employing curable orcross-linkable materials.
 15. A method for the surgical implantation ofa prosthesis comprising the steps of:topographically scanning a regionof anatomical pathology; inputting scanning data so obtained into a datastorage means; computing the stored scanning data according to apredetermined algorithm to reconstruct a plurality of parallel twodimensional cross-sectional images of the anatomical pathology;computing said plurality of two dimensional cross-sectional imagesaccording to a predetermined algorithm to generate a three dimensionalco-ordinate data set for the anatomical pathology; generating a threedimensional representation of said anatomical pathology or a mouldsurface therefor by a constructive modelling process using selectedsequential two dimensional image data sets computed from said threedimensional co-ordinate data set, said two dimensional image data setsbeing computed in parallel planes which are not parallel to the planesin which said two dimensional cross-sectional images are obtained byscanning; and surgically implanting in a patient a prosthesis obtaineddirectly or indirectly therefrom, said prosthesis being characterised inhaving a close fit with connective tissue and contours appropriate foran implant site.
 16. A method as claimed in claim 15 wherein said twodimensional image data sets are computed in parallel planes at closerintervals than planes in which said two dimensional cross sectionalimages are obtained by scanning.
 17. A method as claimed in claim 15wherein said three dimensional representation includes a region ofanatomical pathology surrounding or adjacent to the region of anatomicalpathology being modelled, said surrounding region providing a templatefor accurate fit of a prosthesis so obtained.
 18. A method as claimed inclaim 15 including the step of simultaneously modelling a first portionof an anatomical pathology with an aperture defining the boundary of adefect and a second portion of said anatomical pathology complementaryto said defect.
 19. A method as claimed in claim 15 wherein said secondportion is modelled with a peripheral boundary slightly larger than theperipheral boundary of said aperture.
 20. A method as claimed in claim15 wherein an anatomical prosthesis is constructed directly from a crosslinkable polymer by a constructive modelling process.
 21. A methodaccording to claim 15 wherein the prosthesis is constructed indirectlyby forming a mould from a constructively modelled anatomical pathologyrepresentation.
 22. A method according to claim 15 wherein theprosthesis is constructed indirectly by manipulation of the scanneddata, the reconstructed two dimensional images or the three dimensionalco-ordinate data set to form a mould or mould surface from which theprosthesis is moulded or otherwise formed.